Generating Geofences

1. A method of generating geofences, comprising: obtaining, by a mobile device, a current location of the mobile device; determining a plurality of nearest locations, the plurality of nearest locations being locations nearest to the current location of the mobile device out of a plurality of stored locations, wherein the mobile device can generate respective local geofences for the plurality of nearest locations; determining a size for a triggering geofence based on a distance between the current location of the mobile device and a particular nearest location that is farthest from the current location of the mobile device of the plurality of determined nearest locations; generating respective local geofences for the plurality of nearest locations except the particular nearest location that is farthest from the current location of the mobile device of the plurality of nearest locations; generating a triggering geofence having the determined size, the triggering geofence enclosing the current location of the mobile device; determining that the mobile device has exited the triggering geofence; and in response to determining that the mobile device has exited the triggering geofence, updating the triggering geofence.

2. The method of claim 1 , wherein updating the triggering geofence comprises: determining a second different size for the triggering geofence, the second different size being based on an updated nearest location that is farthest from a second current location of the mobile device of a plurality of second nearest locations.

3. The method of claim 1 , wherein determining a plurality of nearest locations comprises determining N nearest locations, and wherein determining the size for the triggering geofence comprises: determining a distance from the current location of the mobile device to an Nth nearest location, the Nth nearest location being farthest from the current location of the mobile device relative to any other of the plurality of nearest locations; and determining the size for the triggering geofence based on the distance from the current location of the mobile device to the Nth nearest location.

4. The method of claim 1 , wherein the mobile device is configured to maintain a maximum of N concurrent geofences, and wherein determining the plurality of nearest locations comprises determining the N nearest locations.

5. A computer program product, encoded on one or more non-transitory computer storage media, comprising instructions that when executed by one or more computers cause the one or more computers to perform operations comprising: obtaining, by a mobile device, a current location of the mobile device; determining a plurality of nearest locations, the plurality of nearest locations being locations nearest to the current location of the mobile device out of a plurality of stored locations, wherein the mobile device can generate respective local geofences for the plurality of nearest locations; determining a size for a triggering geofence based on a distance between the current location of the mobile device and a particular nearest location that is farthest from the current location of the mobile device of the plurality of determined nearest locations; generating respective local geofences for the plurality of nearest locations except the particular nearest location that is farthest from the current location of the mobile device of the plurality of nearest locations; generating a triggering geofence having the determined size, the triggering geofence enclosing the current location of the mobile device; determining that the mobile device has exited the triggering geofence; and in response to determining that the mobile device has exited the triggering geofence, updating the triggering geofence.

6. The computer program product of claim 5 , wherein updating the triggering geofence comprises: determining a second different size for the triggering geofence, the second different size being based on an updated nearest location that is farthest from a second current location of the mobile device of a plurality of second nearest locations.

7. The computer program product of claim 5 , wherein determining a plurality of nearest locations comprises determining N nearest locations, and wherein determining the size for the triggering geofence comprises: determining a distance from the current location of the mobile device to an Nth nearest location, the Nth nearest location being farthest from the current location of the mobile device relative to any other of the plurality of nearest locations; and determining the size for the triggering geofence based on the distance from the current location of the mobile device to the Nth nearest location.

8. The computer program product of claim 5 , wherein the mobile device is configured to maintain a maximum of N concurrent geofences, and wherein determining the plurality of nearest locations comprises determining the N nearest locations.

9. A system comprising one or more computers operable to perform operations comprising: obtaining, by a mobile device, a current location of the mobile device; determining a plurality of nearest locations, the plurality of nearest locations being locations nearest to the current location of the mobile device out of a plurality of stored locations, wherein the mobile device can generate respective local geofences for the plurality of nearest locations; determining a size for a triggering geofence based on a distance between the current location of the mobile device and a particular nearest location that is farthest from the current location of the mobile device of the plurality of determined nearest locations; generating respective local geofences for the plurality of nearest locations except the particular nearest location that is farthest from the current location of the mobile device of the plurality of nearest locations; generating a triggering geofence having the determined size, the triggering geofence enclosing the current location of the mobile device; determining that the mobile device has exited the triggering geofence; and in response to determining that the mobile device has exited the triggering geofence, updating the triggering geofence.

10. The system of claim 9 , wherein updating the triggering geofence comprises: determining a second different size for the triggering geofence, the second different size being based on an updated nearest location that is farthest from a second current location of the mobile device of a plurality of second nearest locations.

11. The system of claim 9 , wherein determining a plurality of nearest locations comprises determining N nearest locations, and wherein determining the size for the triggering geofence comprises: determining a distance from the current location of the mobile device to an Nth nearest location, the Nth nearest location being farthest from the current location of the mobile device relative to any other of the plurality of nearest locations; and determining the size for the triggering geofence based on the distance from the current location of the mobile device to the Nth nearest location.

12. The system of claim 9 , wherein the mobile device is configured to maintain a maximum of N concurrent geofences, and wherein determining the plurality of nearest locations comprises determining the N maximum number of nearest locations.